Mercury cycling in low alkalinity lakes and factors influencing bioavailability

The goal of this research is to improve our understanding of the dynamic behavior of mercury in lakes and lake food webs. There are three parts to the research. The first part is an assessment of mercury concentrations in northern Minnesota lakes. State-of-the-science methods for sample collection and measurement for ultratrace levels of mercury species are used. Concentrations of total mercury and methylmercury are measured in water and zooplankton. This database of water and zooplankton mercury concentrations are statistically analyzed for seasonal patterns and for correlations among mercury and other water chemistry variables. The second part of the research are laboratory experiments to measure binding of mercury to dissolved organic matter, using lakewater and commercial humic acid. The binding experiments are used to show the relationship between labile (unextracted) and total (extracted) methylmercury. Based on the results of the DOC-Hg binding experiments, inferences are made regarding the relationship between dissolved organic carbon and bioaccumulation of mercury. The third part of the research includes laboratory experiments to measure uptake of mercury in Daphnia magna, in which all aspects (e.g., food, water chemistry, temperature, light) are controlled. Culturing of D. magna and the experimental design follow U.S. EPA guidelines for toxicology experiments. In the experiments the influences of organic carbon concentration and ionic strength on mercury uptake are tested, as well as the relative importance of uptake from food and water. The three parts of the research are used to develop a mechanistic explanation for the bioavailability of mercury in aquatic systems.; The combined results of the laboratory experiments and field monitoring demonstrated the predominant importance of DOC as the controlling factor influencing Hg speciation and bioavailability. Alkalinity appears to influence Hg speciation and bioavailability because of its effect on DOC rather than a direct effect on Hg species. DOC has a complex relationship with Hg in that it can transport high concentrations of Hg, which increases the potential for greater Hg exposure to the aquatic biota, while at the same time DOC can reduce exposure at low MeHg concentrations through complexation.